1. Field of the Invention
The invention relates to a device for reducing exhaust gas contaminants, particularly for motor vehicles, which includes a catalyst body disposed within a housing such that the periphery of the catalyst body is spaced from the housing, and an intumescent mat disposed within a gap formed between the housing and the catalyst body.
2. Description of the Related Art
Such devices, hereinafter simply called "catalytic converters", are essentially composed of a usually dual-shell housing of a metallic material in which one or also several catalyst bodies are mounted. The catalyst body is penetrated by channels that extend parallel to the direction of flow of the exhaust gases and are coated with a catalytically active material. In the mentioned catalytic converters, the catalyst body is frequently composed of a ceramic material. Such ceramic bodies, the so-called monoliths, are naturally very sensitive to breaking and, compared to metallic materials, have a negligible coefficient of thermal expansion.
The monoliths are mounted in the catalytic converter housing by means of conventional supporting mats, usually called intumescent mats. These intumescent mats are mounted with excess dimensions compared to the free width of the gap between the monolith and the housing wall, thus resulting in a pressure force that is generated by the elastic restoring forces of the mineral fiber binder system on the housing wall and the peripheral surface of the monolith. The expansion particles embedded in the intumescent mat, for example in the form of hydrated mica platelets, now have the characteristic that their volume increases by a multiple beginning at a temperature of about 400.degree. C. The reason for this increase in volume is that the water bound in the intermediate layers of the hydrated mica is irreversibly split off, changes to the vapor state and expands the stacked mica layers somewhat in the manner of an accordion. The water vapor is then caught in the pockets formed by the spread-apart mica layers. Although the volume of the bloated mica platelets is reduced to a certain degree when the system cools to room temperature due to the reduction in pressure and a partial escape of the water vapor that is trapped in the pockets, the surface pressure exerted by the intumescent mat on the housing wall and the monolith is increased considerably compared to the state before the first use of the catalytic converter. When the intumescent mat is again heated to operating temperature, the expanded mica platelets, due to the expansion of the gas or water vapor enclosed in the pockets, act like gas springs.
Due to the described characteristics of the expansion particles, the increase resulting from the different coefficients of thermal expansion of the housing material and the monolith in the size of the gap disposed between the housing and the monolith is compensated. In the case of intumescent mats to which a binder has been added in order to increase their initial elasticity, the thermally caused reduction of the elastic restoring force of the intumescent mat as a result of the heat is additionally compensated. At the existing operating temperatures, cracking and oxidative processes decompose or reduce the binder, which is generally composed of a rubber-elastic synthetic or natural material, in the intumescent mat with at least a partial loss of its mass and its elastic recovery property.
The drawback of the known intumescent mats is now that the desired expanded state occurs only if the vehicle is driven at least once in a load range in which the heat supplied to the intumescent mat by the exhaust gases is sufficient to increase the temperature to at least 400.degree. C. If such an operating state is not attained over a longer period of time, the carrier body may, under unfavorable circumstances, for example with poor road conditions or jerky driving, may come loose from its attachment and may be at least partially destroyed when it hits metal components of the catalytic converter housing. This effect may occur, for example, in vehicles having large displacement engines and, in particular, if in addition, due to local traffic rules, relatively low speed limits are prescribed or if vehicles equipped with small or medium size engines are driven by persons in a restrained manner.